Strong magnetic fields with the magnetic field strength having a high degree of stability over time are required, for example, for nuclear magnetic resonance imaging. Electromagnets with superconducting coils have been developed for this purpose. Coils such as these have been known for several decades, composed of low-temperature (LTc) superconductor material such as niobium-tin or niobium-titanium. Magnets such as these can be operated in the temperature range around about 4 K.
For about the last decade, superconducting materials of the high-temperature type (HTc superconductors) have also been known, which are superconducting up to liquid air temperatures. For example, these superconducting materials remain superconducting at temperatures below 77 K. Electromagnets with HTc-superconducting coils have also already been produced, which may be used for strong magnetic fields, for example up to temperatures below about 40 K. This relatively low operating temperature is due to the fact that the HTc current capacity of the HTc superconductor materials used for this purpose, for example bismuth cuprate (Bi, Pb) 2Sr2Ca2Cu3O10 and Bi2Sr2CaCu2O8 and rare-earth cuprates RE Ba2Cu3O7 where RE=Nd, Gd, Sm, Er, Y, is sufficient only up to an operating temperature which is limited as a function of the magnitude of the magnetic field that exists.
In an ideal situation, a short-circuit superconducting current produced and flowing in such a superconducting coil of a magnet lasts indefinitely. A device known as a flux pump is used, for example, for feeding such a superconducting current into a superconductor coil. One such flux pump is known, for example, from “Study of Full-Wave Superconducting Rectifier-Type Flux-Pumps” in IEEE Transactions on Magnetics, Vol. 32 (1996) pp. 2699–2702 and from “On Fully Superconducting Rectifiers and Flux Pumps”, Cryogenics, May 1991, pages 262–275.
The indicated articles relate exclusively to superconductors of the low-temperature (LTc) type, that is to say to materials such as niobium-tin and niobium-titanium which have been mentioned. FIG. 1 shows an example of a flux pump 2 of the rectifier type from the prior art (from IEEE Transactions . . . as above), in which 11 denotes the superconducting coil with an LTc superconductor of an electromagnet 111, such as that which is known to be used, for example, for the already mentioned nuclear magnetic resonance imaging. Reference numeral 12 denotes a power source, which supplies the electrical power resulting in the formation of the superconducting current that flows in the coil 11 during operation of the electromagnet. Reference numeral 13 denotes a transformer with a primary coil 113 and, in this example, reference numeral 2 denotes secondary coils 213 and 313 connected in series. Reference numerals 15 and 16 denote two switches for making and breaking the circuit for the superconducting current flowing in the circuit of the respective secondary coil 213 or 313.
In the prior art, these two secondary coils and switches are composed of LTc-superconducting material while, in the invention which is still to be described, they are composed of HTc-superconducting material. In order to be able to operate as a transformer 13, the power source generally annotated with reference numeral 12 supplies an alternating current, that is to say a current with a current direction which is repeatedly and successively reversed. The switches 15 and 16 are opened and closed in time with this current direction change, to be precise in the opposite sense to one another. This results in rectification of the electric current flowing through the lines that are annotated with reference numerals 20 and 21. This current is the supply current for the coil 11 of the electromagnet. Reference numeral 23 denotes a known protection device, which will not be described in detail, for protection of the flux pump 2. Reference numeral 25 denotes a control system for controlling the timing of the changes in the supply current from the power source 12 and the switches 15 and 16.
In the known flux pump shown in FIG. 1, the switches 15 and 16 are low-temperature (LTc) superconductor switches. Their “open” and “closed” states are produced by the “superconducting” or “normally conductive” states of the conductor material contained in them. The superconducting state occurs when cooled to a sufficiently low temperature. Heating the respective switch element changes it to the normally conductive state, which corresponds to an open switch. This conversion is reversible.
The coil 11 of the electromagnet and its circuit can be successively charged with superconducting current in a manner known as periodic switching of the switches 15 and 16, so that a corresponding constant electromagnet field with a strong magnetic field strength and a high magnetic flux is correspondingly produced successively in the coil 11 of the electromagnet, and its permanently provided superconduction is maintained. This permanence for the LTc superconduction and the materials used for this purpose and which have already been mentioned above is true to a large extent. For example, once a superconductor electromagnet has been charged, for example in a nuclear magnetic resonance imager, its magnetic field strength remains constant over a sufficiently long time that the extremely stringent requirements for constancy of the field for nuclear magnetic resonance imaging are satisfied by this magnetic field. Recharging is required, for example, only after about 100 hours, provided no technical defects or operating errors occur.
The fundamental principles of these known flux pumps are applicable only, to a limited extent, to the use of high-temperature HTc superconducting materials provided according to the present invention. For projects and apparatuses according to the present invention, which use such materials, it is often necessary to take into account particular or different conditions or circumstances.